The goal of the research is to describe the neural circuits that provide input to the midget and parasol ganglion cells that give rise to the two major parallel processing streams in the macaque visual pathway. Because the retina of macaque monkey is very similar to that of humans, the results will also be helpful for understanding human vision. Specific aim 1 investigates midget cells that are known to project to the parvocellular layers of the LGN, and are thought to contribute to spatial vision, and to red-green color vision. Their inputs from long and medium wavelength cones via midget bipolar cells have been well characterized in the central retina. During the last funding period, inputs to midget ganglion cells were investigated in peripheral retina as well, and two amacrine cells in this pathway were described and will be identified in the proposed experiments. The working hypothesis is that interactions between these two amacrine cells are essential to account for the color-specific surround responses of midget ganglion cells. Specific aim 2 investigates parasol ganglion cells that project to the magnocellular layers of the LGN and contribute to perception of motion as well as other aspects of perception. During the prior funding period, synaptic inputs to parasol cells were described, and gap junctions were found at two sites providing input to these cells. The working hypothesis is that gap junctions are a common feature of this pathway, and the proposed experiments will identify more types of presynaptic neurons and examine potential sites of electrical coupling. Specific aim 3 is to make a realistic computer model of the neural circuits that were found in anatomical studies to provide input to ganglion cells. The model of the input circuit to midget ganglion cells will test the novel hypothesis that accounts for their cone-specific surrounds without cone-specific connections except those known to exist in central retina between cones, midget bipolar cells and midget ganglion cells. Parasol cells are more sensitive than midget cells to luminance contrast and respond more transiently. Unlike midget cells, they also fire in synchrony with other parasol cells of the same subtype. The modeling studies will test hypotheses that differences in synaptic inputs to the midget and parasol ganglion cells can account for many of these physiological differences.